Power converter with compensation circuit for adjusting output current provided to a constant load

ABSTRACT

A power converter for constant loads includes an energy transfer element, a switch, a controller, and a compensation circuit. The energy transfer element is coupled to receive a rectified voltage having a blocked portion that corresponds to a phase angle. The controller is coupled to control switching of the switch to regulate an output current of the power converter in response to a plurality of signals. The plurality of signals includes a peak input voltage signal and a feedback signal. The compensation circuit is coupled to output a compensation signal to adjust at least one of the plurality of signals in response to the phase angle exceeding a phase angle threshold. The compensation circuit does not output the compensation signal and does not adjust the at least one of the plurality of signals when the phase angle is less than the phase angle threshold.

REFERENCE TO PRIOR APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/824,071, filed Jun. 25, 2010 now U.S. Pat. No. 8,294,377. U.S. patentapplication Ser. No. 12/824,071 is hereby incorporated by reference.

TECHNICAL FIELD

Embodiments of the present invention relate generally to power supplies,and more specifically to power supplies utilized with dimmer circuits.

BACKGROUND INFORMATION

Electronic devices are typically used with power conversion circuits.Switched mode power converters are commonly used due to their highefficiency, small size and low weight to power many of today'selectronics. Conventional wall sockets provide a high voltagealternating current. In a switching power converter a high voltagealternating current (ac) input is converted to provide a well regulateddirect current (dc) output. In operation, a switch, included in thepower converter, is utilized to control the desired output current byvarying the duty cycle (typically the ratio of the on time of the switchto the total switching period) and/or varying the switching frequency(the number of switching events per unit time). More specifically, apower converter controller may determine the duty factor and/orswitching frequency of the switch in response to a measured inputvoltage and a measured output voltage.

In one type of dimming for lighting applications, a dimmer circuittypically blocks a portion of the ac input voltage to limit the amountof power supplied to an incandescent lamp. Particularly, the dimmercircuit outputs a dimmer output voltage that is representative of an acinput voltage with portions of the ac input voltage removed or blockedto provide a dimming function. The removing or blocking of a portion ofthe ac input voltage may be referred to as phase dimming because it isoften convenient to designate the position of the missing voltage of thedimming output voltage in terms of a fraction of the period of the acinput voltage measured in degrees. In general, the ac input voltage is asinusoidal waveform and the period of the ac input voltage is referredto as a full line cycle. As such, half the period of the ac inputvoltage is referred to as a half line cycle. An entire period has 360degrees, and a half line cycle has 180 degrees. Typically, the phaseangle is a measure of how many degrees (from a reference of zerodegrees) of each half line cycle the dimmer circuit blocks. As such,removal of half the ac input voltage in a half line cycle by the dimmercircuit corresponds to a phase angle of 90 degrees.

LED (light emitting diode) loads require a regulated power converter toprovide regulated current from the ac power line. A controller includedin the conventional regulated power converter may measure the inputvoltage in order to regulate the output of the power converter.Generally, a common way to measure the input voltage is to sense a peakof the ac input voltage. In certain configurations, a dimmer circuit maybe coupled between an ac source providing the ac input voltage and thesense circuit that may be used to sense the peak input voltage.Therefore, the power converter may rely on sensing an output of thedimmer circuit, the dimmer output voltage, which may still ‘capture’ or‘hold’ the peak of the ac input voltage. Thus, natural dimming of theLED load may occur when the peak of the dimmer output voltage equals acorresponding peak input voltage of the ac power line. This is becausethe controller may not realize that a portion of the ac input voltagehas been blocked by a dimmer circuit, thereby reducing the amount ofpower delivered to the output. However, when a phase angle is greaterthan 90 degrees, the dimmer circuit may block the peak input voltage andtherefore misrepresent the peak input voltage with a lower voltage. Inthis situation, a controller included in the conventional regulatedpower converter is likely to prevent natural dimming because thecontroller is receiving a peak dimmer output voltage that should berepresentative of the peak input voltage but is instead representativeof a voltage less than the peak ac input voltage. To further explain, acontroller included in a conventional regulated power converter maysense the peak dimmer output voltage that is the peak dimmer outputvoltage that corresponds to a voltage along the ac line that is lessthan the peak ac input voltage and thus detects less incoming power atthe input of the power converter. The controller may then increase dutycycle and/or switching frequency and inadvertently increase powerdelivery to the output. As a result a controller included in aconventional regulated power converter may actually prevent or ‘fight’against natural dimming of the led load.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of severalembodiments of the present invention will be more apparent from thefollowing more particular description thereof, presented in conjunctionwith the following drawings.

FIG. 1 is a functional block diagram illustrating an example dimmercircuit, a constant load, and a switching power converter, in accordancewith an embodiment of the present invention.

FIG. 2A is a diagram illustrating an example input voltage waveform, inaccordance with an embodiment of the present invention.

FIG. 2B is a diagram illustrating an example dimmer output voltagewaveform, in accordance with an embodiment of the present invention.

FIG. 2C is a diagram illustrating an example rectified voltage waveform,in accordance with an embodiment of the present invention.

FIG. 3A is a diagram illustrating an LED array, in accordance with anembodiment of the present invention.

FIG. 3B is a diagram illustrating a circuit model of LEDs included inthe LED array of FIG. 3A.

FIG. 3C is a graph illustrating a relationship between output currentand output voltage of the circuit model of LEDs of FIG. 3B.

FIG. 4 is a functional block diagram of an LED array and a switchingpower converter, in accordance with an embodiment of the presentinvention.

FIG. 5 is a flowchart illustrating operation of the switching powerconverter of FIG. 4.

FIG. 6 is a functional block diagram of an LED array and a switchingpower converter, in accordance with an embodiment of the presentinvention.

FIG. 7 is a flowchart illustrating operation of the switching powerconverter of FIG. 6.

DETAILED DESCRIPTION

Embodiments of a power converter with a compensation circuit foradjusting output current provided to a constant load are describedherein. In the following description numerous specific details are setforth to provide a thorough understanding of the embodiments. Oneskilled in the relevant art will recognize, however, that the techniquesdescribed herein can be practiced without one or more of the specificdetails, or with other methods, components, materials, etc. In otherinstances, well-known structures, materials, or operations are not shownor described in detail to avoid obscuring certain aspects.

Reference throughout this specification to “one embodiment”, “anembodiment”, “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment or example is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment”,“in an embodiment”, “one example” or “an example” in various placesthroughout this specification are not necessarily all referring to thesame embodiment or example. Furthermore, the particular features,structures or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. In addition, it is appreciated that the figures providedherewith are for explanation purposes to persons ordinarily skilled inthe art and that the drawings are not necessarily drawn to scale.

For phase dimming applications, including those for a constant load(e.g., light emitting diodes (LEDs)), a phase dimmer circuit typicallyblocks a portion of the ac input voltage at every half line cycle tolimit the amount of power supplied to the LEDs. As mentioned above, thephase angle may be a measure of how many degrees of each half line cycleof the ac input voltage the dimmer circuit blocks. For example, the halfline cycle of the ac input voltage may have a total of 180 degrees. Assuch, removal of half the ac input voltage in a half line cycle by thedimmer circuit corresponds to a phase angle of 90 degrees. In anotherexample, removal of a quarter of the ac input voltage in a half linecycle may correspond to a phase angle of 45 degrees. A triac dimmercircuit is one example of a phase dimmer circuit.

For embodiments of the present invention, a power converter controllercontrols switching of a switch to regulate an output current in responseto a plurality of signals. These signals may include an input voltagesignal representative of a peak input voltage of the power converter anda feedback signal representative of an output voltage of the powerconverter. A power converter controller in accordance with oneembodiment may be designed to measure the ac input voltage by measuringthe peak of the ac input voltage and adjusting a duty cycle of a switchto maintain regulation of the output current. In certain configurations,the power converter may not be able to directly measure the ac inputvoltage due to a dimmer circuit coupled between the ac source thatprovides the ac input voltage and a sense circuit that will sense theinput voltage. In one example, the power converter may sense a peak ofthe dimmer output voltage to be representative of the peak inputvoltage. Under certain conditions (phase angle <90 degrees), a peakdimmer output voltage is substantially equal to the peak input voltage.In other words, this allows the power converter to measure the peakdimmer output voltage as an indirect way to measure the peak inputvoltage.

In operation, as the peak input voltage drops (indication of the inputvoltage dropping), the reduced peak input voltage may cause thecontroller to extend the duty cycle or increase switching frequency inan attempt to maintain the regulated output current. In one example whena phase angle exceeds 90 degrees, a peak dimmer output voltage may nolonger represent the peak input voltage. More specifically, when thephase angle exceeds 90 degrees, the peak dimmer output voltage is lessthan the peak input voltage. In this instance, the controller attemptsto adjust its power delivery to maintain output regulation with theassumption that the power converter is receiving a lower input voltage.This results in the controller fighting or resisting the dimming becausethe controller may inadvertently deliver more power than intended, tothe output. Accordingly, embodiments of the present invention include acompensation circuit to adjust at least one of these signals received bythe controller (e.g., the input voltage signal and/or the feedbacksignal) to ensure the controller does not inadvertently deliver morepower. With this feature, natural diming may extend even when more than90 degrees of the ac input voltage wave form is blocked off by thedimming circuit. In one example, this feature may allow for a greaterdimming range.

Referring first to FIG. 1, a functional block diagram of an exampledimmer circuit 105, a constant load 110, and a switching power converter100 is illustrated. Switching power converter 100 is illustrated asincluding a rectifier 115, an energy transfer element 120, a switch 125,an input return 130, a rectifier 135, an output capacitor 140, an outputreturn 145, an output sensor 150, a controller 155, a compensationcircuit 160, a summer 165, and a peak input voltage detector 170. Energytransfer element 120 is illustrated as including a primary winding 175and a secondary winding 180. Also shown in FIG. 1, are ac input voltageV_(IN) (may be referred to as ‘input voltage V_(IN)’), dimmer controlinput 104, dimmer output voltage V_(DO), rectified voltage V_(RECT),output current I_(OUT), output voltage V_(OUT), feedback signal U_(FB),compensation signal U_(COMP), peak input voltage signal U_(VINPK), andcompensated peak input voltage signal U_(CVINPK).

Switching power converter 100 provides a regulated output currentI_(OUT) to load 110 from an unregulated ac input voltage V_(IN). Asshown, FIG. 2A is a diagram illustrating an example input voltagewaveform of ac input voltage V_(IN), in accordance with an embodiment ofthe present invention.

Referring back to FIG. 1, dimmer circuit 105 receives input voltageV_(IN) and produces dimmer output voltage V_(DO). In one embodiment,dimmer circuit 105 is coupled to receive dimming control input 104.Dimming control input 104 is representative of an amount of inputvoltage V_(IN) that is to be blocked or ‘chopped off’ by dimmer circuit105. For example, in a typical household application, dimming controlinput 104 may be received by way of manual manipulation of a wallmounted dimmer intended to vary the brightness of a light. In anotherexample, dimming control input 104 may be, received by way of a digitalcontrol system, such as in a home automation system. In one embodiment,dimmer circuit 105 is a phase dimming circuit such as a triac phasedimmer.

In operation, depending on the amount of dimming indicated by dimmingcontrol input 104, dimmer circuit 105 controls the amount of time thatinput voltage V_(IN) is disconnected from power converter 100. Ingeneral, the more dimming demanded corresponds to a longer period oftime ac input voltage V_(IN) is blocked. As shown, FIG. 2B is a diagramillustrating an example dimmer output voltage waveform, in accordancewith an embodiment of the present invention. The dimmer output voltagewaveform illustrated in FIG. 2B is one possible representation of dimmeroutput voltage V_(DO) of FIG. 1.

Referring back to FIG. 1, dimmer circuit 105 further couples torectifier 115 such that the dimmer output voltage V_(DO) is received byrectifier 115. Rectifier 115 outputs rectified voltage V_(RECT). In oneembodiment, rectifier 115 is a full-wave bridge rectifier. FIG. 2C is adiagram illustrating an example rectified voltage waveform, inaccordance with an embodiment of the present invention. The rectifiedvoltage waveform illustrated in FIG. 2C is one possible representationof rectified voltage V_(RECT) 108 of FIG. 1.

FIG. 1 further illustrates rectifier 115 as coupled to energy transferelement 120. Energy transfer element 120 may be a transformer includingprimary winding 175 and secondary winding 180. However, it should beappreciated that the energy transfer element 120 may have more than twowindings. Primary winding 175 is further coupled to switch 125, which isthen further coupled to input return 130. In one embodiment, the switch125 may be a transistor such as a metal-oxide-semiconductor field-effecttransistor (MOSFET). In one example, controller 155 and switch 125 couldform part of an integrated control circuit that is manufactured aseither a hybrid or monolithic integrated circuit.

As shown, secondary winding 180 of energy transfer element 120 iscoupled to the rectifier 135. In the example of FIG. 1, the rectifier135 is a diode. Both output capacitor 140 and constant load 110 areshown in FIG. 1 as being coupled to the rectifier 135. In operation,current through rectifier 135 is filtered by the output capacitor 140 toproduce a substantially constant output current I_(OUT) which isreceived by constant load 110.

Constant load 110 may maintain a substantially constant power draw whenno dimming is indicated. Constant load 110 may also be a load where theoutput voltage varies as a function of the output current in apredetermined and known manner. For example, output voltage V_(OUT) maybe substantially proportional to I_(OUT). In one embodiment, constantload 110 may be a light emitting diode (LED) array, as will be discussedfurther.

Output sensor 150 is coupled to sense output voltage V_(OUT) from theoutput of switched mode power converter 100 and to produce the feedbacksignal U_(FB). Output sensor 150 is further coupled to controller 155and compensation circuit 160 such that the feedback signal U_(FB) isreceived by both the controller 155 and by compensation circuit 160.Feedback signal U_(FB) may be a voltage signal or a current signal andis representative of output voltage V_(OUT). In one embodiment, feedbacksignal U_(FB) received by controller 155 is a current signal whilefeedback signal U_(FB) received by compensation circuit 160 is a voltagesignal.

As shown in FIG. 1, peak input voltage detector 170 is coupled torectifier 115 to detect the peak voltage V_(PK) of rectified voltageV_(RECT) 108. Peak input voltage detector 170 is also coupled to providethe peak input voltage signal U_(VINPK), which may be representative ofthe peak ac input voltage V_(PK).

Controller 155 is coupled to generate a drive signal 138 to control theswitching of switch 125. Controller 138 may be implemented as amonolithic integrated circuit or may be implemented with discreteelectrical components or a combination of discrete and integratedcomponents. In addition, switch 125 receives the drive signal 138 fromthe controller 155.

Switch 125 is opened and closed in response to drive signal 138. It isgenerally understood that a switch that is closed may conduct currentand is considered on, while a switch that is open cannot substantiallyconduct current and is considered off. In operation, the switching ofswitch 125 produces a pulsating current at the rectifier 135.

As shown, controller 155 outputs drive signal 138 to control theswitching of switch 125 in response to the feedback signal U_(FB) and inresponse to the peak input voltage signal U_(VINPK). In one embodiment,the drive signal 138 may be a PWM (pulse width modulated) signal oflogic high and logic low sections, with the logic high valuecorresponding to a closed switch and a logic low corresponding to anopen switch. In another embodiment, the drive signal may be comprised ofsubstantially fixed-length logic high (or ON) pulses and regulates byvarying the number of ON pulses over a set time period.

In operation, drive signal 138 may have various drive signal operatingconditions such as the switch on-time t_(ON) (typically corresponding toa logic high value of the drive signal 138), switch off-time t_(OFF)(typically corresponding to a logic low value of the drive signal 138),switching frequency f_(S), or duty cycle. As mentioned above, load 110is a constant load. Thus, during operation, controller 155 may utilizefeedback signal U_(FB) and peak input voltage signal U_(VINPK) toregulate output current I_(OUT). For example, a reduction in the peakinput voltage signal U_(VINPK)% may correspond to a lower ac inputvoltage V_(IN). Thus, controller 155 may extend the duty cycle of drivesignal 138 to maintain a constant output current I_(OUT) in response tothis reduction in the peak input voltage signal U_(VINPK). However, asdiscussed above, the peak input voltage signal U_(VINPK) may also beinadvertently reduced when a dimmer circuit is used. For example, FIG.2C illustrates an example rectified voltage waveform of rectifiedvoltage 108 for varying phase angles θ (i.e., theta). The first twohalf-line cycles of the rectified voltage waveform correspond to a phaseangle of zero degrees, or in other words, no dimming is desired. Forthese first two half-line cycles, the peak voltage sensed by peak inputvoltage detector 170 is substantially equal to peak voltage V_(PK).

The next two half-line cycles of FIG. 2C correspond to a phase angle of60 degrees (i.e., θ₁=60 degrees). In this example, again, a peak of therectified voltage V_(RECT) sensed by peak input voltage detector 170 issubstantially equal to peak voltage V_(PK), which is representative of amagnitude of a peak of input voltage V_(IN). While controller 155regulates output current I_(OUT) based, in part, on the peak voltageV_(PK) indicated by peak input voltage signal U_(VINPK), a portion of acinput voltage V_(IN) has been removed by dimmer circuit 105. Forexample, the waveform illustrated in FIG. 2C shows a non-blocked portion210 and a blocked portion 205 of the waveform, where an amount ofblocked portion 205 corresponds to a phase angle of 60 degrees. Thus,the output voltage V_(OUT) 114 and output current I_(OUT) 112 arereduced due to the reduction in power transfer from primary winding 175to secondary winding 180. Although FIG. 2C illustrates θ₁=60 degrees, itis recognized that for phase angles equal to or less than 90 degrees amagnitude of a peak of the rectified voltage V_(RECT) sensed by peakinput voltage detector is substantially equal to a magnitude of acorresponding peak of input voltage V_(IN).

The last two half-line cycles of FIG. 2C correspond to a phase angle of120 degrees (i.e., θ₂=120 degrees). In this example, the waveformillustrated in FIG. 2C shows a non-blocked portion 220 and a blockedportion 215 of the waveform, where an amount of blocked portion 215corresponds to a phase angle of 120 degrees. In this situation, the peakvoltage sensed by peak input voltage detector 170 is substantially equalto a peak voltage V′_(PK) and is less than a magnitude of a peak voltageV_(PK), which would be substantially equal to the peak of input voltageV_(IN). Although FIG. 2C illustrates θ₂=120 degrees, it is recognizedthat for phase angles greater than 90 degrees, a peak of rectifiedvoltage V_(RECT) sensed by peak input voltage detector 170 is less thana corresponding magnitude of a peak of input voltage V_(IN) representedby peak voltage V_(PK). As discussed above, sensing a lower voltage (i.epeak voltage V′_(PK)) will cause controller 155 to increase powerdelivery in an attempt to maintain the regulated output current I_(OUT)112. This is counter to the desire of natural dimming a constant load,such as an LED array. Accordingly, power converter 100 includescompensation circuit 160 and summer 165 to adjust the peak input voltagesignal U_(VINPK) so that output current I_(OUT) delivered to load 110 isreduced (e.g., adjusted) with the increase demand of dimming (i.e.,increase of θ) to allow for “natural dimming” at the output.

In one embodiment, compensation circuit 160 is coupled to summer 165 toadjust the peak input voltage signal U_(VINPK) when the phase angleexceeds a phase angle threshold. For example, referring back to FIG. 2C,the phase angle threshold may be equal to 90 degrees, such that when thephase angle θ is greater than 90 degrees, compensation circuit 160adjusts the peak input voltage signal U_(VINPK). Further with thisexample, when the phase angle θ is equal to or less than 90 degrees,compensation circuit 160 does not adjust the peak input voltage signalU_(VINPK).

As discussed above, compensation circuit 160 receives feedback signalU_(FB), which is representative of output voltage V_(OUT). Thus, as theoutput voltage V_(OUT) is reduced due to an increased demand for dimmingindicated by dimmer control input 104 resulting in more of the ac inputvoltage V_(IN) being blocked, the feedback signal U_(FB) will also bereduced. In one embodiment, compensation circuit 160 includes a phaseangle voltage threshold which is representative of the phase anglethreshold to determine when to generate compensation signal U_(COMP). Inparticular, the phase angle threshold is representative of a phase angleat which compensation signal U_(COMP) may assist in reducing powerdelivery to output of power converter. A phase angle voltage thresholdis representative of an output voltage that corresponds with aparticular phase angle threshold. When feedback signal U_(FB), which isrepresentative of the output voltage V_(OUT), is less than the phaseangle voltage threshold, compensation circuit 160 generates compensationsignal U_(COMP). Summer 165 then adds compensation signal U_(COMP) tothe peak input voltage signal U_(VIN) to provide a compensated inputvoltage signal U_(CVINPK) to controller 155. In one embodiment, whenphase angle θ is greater than 90° the compensated peak input voltagesignal U_(CVINPK) is representative of the peak voltage V_(PK) of peakinput voltage V_(IN) even though the peak input voltage signal U_(VINPK)indicates a sensed voltage representative of peak voltage V′_(PK) whichis less than the peak voltage V_(PK). When controller 155 receives thecompensated peak input voltage signal U_(CVINPK), the controller 155 maynot attempt to increase output power to maintain output current I_(OUT).Thus, output current I_(OUT) is allowed to naturally reduce due to thereduction in power transfer from primary winding 175 to secondarywinding 180. In one example, the phase angle voltage threshold used bycompensation circuit 160 is representative of a dimmer control input 104corresponding to a phase angle of 90 degrees. Thus, in this example,compensation signal U_(COMP) is only output from compensation circuit160 for phase angles greater than 90 degrees.

As discussed above, constant load 110 may be a substantially constantload that does not vary during operation of the power converter. Thus,FIG. 3A illustrates an LED array 305, which is one possibleimplementation of constant load 110 of FIG. 1. As shown, LED array 305includes N number of LEDs (i.e., LED 1 though LED N). As further shown,FIG. 3B is a diagram illustrating a circuit model of LEDs included inthe LED array 305 of FIG. 3A. LEDs 301, 302, 303, and 304 are “ideal”circuit models of LEDs 1, 2, 3, and N, respectively, of FIG. 3A. Thatis, LED 1 may be represented by ideal LED 301 which includes a diode D₁,a threshold voltage V_(D1) and a series resistance R_(S1). Thus, LED 301will generally conduct current when the voltage across LED 301 exceedsthreshold voltage V_(D1) and the current through LED 301 will beproportional to the voltage across it due to series resistance R_(S1).FIG. 3C is a graph illustrating a relationship between output currentand output voltage of the circuit model of LEDs of FIG. 3B. As shown inFIG. 3C, the sum of the threshold voltages V_(D1) through V_(DN)represents a minimum voltage V_(MIN) necessary to turn on the LEDs. Thatis, LED array 305 will generally not conduct current until the outputvoltage V_(OUT) exceeds the minimum voltage V_(MIN). Also, shown in FIG.3C is that for output voltages V_(OUT) greater than the minimum voltageV_(MIN), the output current is generally proportional to the outputvoltage V_(OUT). In other words, as the output current I_(OUT) isreduced through LED array 305 a proportional reduction in voltage acrossthe series resistance R_(S1), R_(S2), . . . R_(SN) occurs as well, thus,reducing the overall output voltage V_(OUT). Thus, controllers inaccordance with the embodiments disclosed herein, may utilize thepredetermined and known relationship between output voltage V_(OUT) andoutput current I_(OUT) to determine a change in the output currentI_(OUT) by sensing the output voltage V_(OUT) rather than directlysensing the output current I_(OUT). In other words, sensing the outputvoltage V_(OUT) may be used to indirectly sense a change in outputcurrent I_(OUT). This allows for compensation signal U_(COMP) to beadjusted in response to output voltage V_(OUT).

FIG. 4 is a functional block diagram of LED array 305 and a switchingpower converter 400, in accordance with an embodiment of the presentinvention. Switching power converter 400 is one possible implementationof power converter 100 of FIG. 1. Switching power converter 400 isillustrated as including a filter capacitor 405, energy transfer element120, input return 130, rectifier 135, output capacitor 140, outputreturn 145, a resistor 445, an output sensor 450, a compensation circuit460, a peak input voltage detector 470, an integrated control circuit480, a bypass capacitor 485, and a summing node 497. Output sensor 450is illustrated as including an auxiliary winding 425, diode 430 andcapacitor 440. Compensation circuit 460 is illustrated as including aresistor divider (i.e., resistors 455 and 465) a resistor 490, atransistor 475, and a node 495. Peak input voltage detector 470 is shownas having a diode 410, a resistor 415, and a capacitor 420. Integratedcontrol circuit 480 is shown as including switch 125 and controller 155.Also shown in FIG. 4 are rectified voltage V_(RECT), output voltageV_(OUT), output current I_(OUT), a feedback current I_(FB), a peak inputvoltage current I_(VINPK), a compensation current I_(COMP), and acompensated peak input voltage signal I_(CVINPK).

As shown in FIG. 4, filter capacitor 405 is coupled to receive therectified voltage V_(RECT). In operation, filter capacitor 405 providesa filtering function for EMI (electro-magnetic interference). In oneexample, controller 115 may perform power factor correction (PFC), wherea switch current I_(SW) through switch 125 is controlled to changeproportionately with the rectified voltage V_(RECT). By way of example,controller 115 may perform power factor correction by controlling theswitching of switch 125 to have a substantially constant duty cycle fora half-line cycle. As such, the value of the filter capacitor 405 may bechosen such that the voltage on the filter capacitor 405 has arelatively low capacitance value.

FIG. 4 illustrates peak input voltage detector 470 as coupled to receivethe rectified voltage V_(RECT) and to generate peak input voltagecurrent I_(VINPK). In one example, peak input voltage current I_(VINPK)is a current representative of a peak of rectified voltage V_(RECT).Peak input voltage detector 470 is one possible implementation of peakinput voltage detector 170 of FIG. 1. As shown in FIG. 4, peak inputvoltage detector 470 includes diode 410, resistor 415 and capacitor 420.In one embodiment diode 410 is coupled to capacitor 420 to preventcapacitor 420 from discharging through primary winding 175, such thatthe sensed peak voltage V_(PKS) is maintained in capacitor 420.

FIG. 4 further illustrates output sensor 450 as including auxiliarywinding 425, diode 430, and capacitor 440. Output sensor 450 is onepossible implementation of output sensor 150 of FIG. 1. As shown in FIG.4, auxiliary winding 425 is magnetically coupled to secondary winding180. In operation, when switch 125 transitions from on to off, switchcurrent I_(SW) is substantially prevented from flowing through powerswitch 125 and the energy stored in input winding 175 is transferred tosecondary winding 180 allowing a voltage across auxiliary winding 425 torepresent a voltage that is proportional to output voltage V_(OUT). Thevoltage across auxiliary winding 425 may be proportional to the outputvoltage V_(OUT) by a proportion of a number of turns in auxiliarywinding 425 to the number of turns in secondary winding 180. Thus,output sensor 450 may generate feedback signal U_(FB) that isrepresentative of the output voltage V_(OUT). The feedback signal U_(FB)received by compensation circuit 460 is a voltage signal, while powerconverter 400 includes resistor 445 to convert the feedback signalU_(FB) to a feedback current I_(FB) that is coupled to be received bycontroller 155 of integrated controller circuit 480. In one embodiment,the value of resistor 445 is selected to set a desired value of theoutput current I_(OUT).

Compensation circuit 460 is coupled to generate a compensation currentI_(COMP) in response to the phase angle of the block portion ofrectified voltage V_(RECT) exceeding the phase angle threshold. Inparticular, compensation circuit 460 may be coupled to generatecompensation current I_(COMP) in response to the feedback signal U_(FB)being less than a phase angle voltage threshold that is representativeof the phase angle threshold. In the illustrated example, the value ofresistors 455 and 465 are selected to set the value of the phase anglevoltage threshold, such that when the voltage at node 495 drops lowenough to turn on transistor 475, the phase angle voltage threshold isreached. In other words, when the phase angle voltage threshold isreached, then the phase angle threshold is also reached. When the phaseangle voltage threshold is reached, the compensation current I_(COMP)flows through transistor 475. In one embodiment, the phase angle voltagethreshold is representative of a dimmer control input corresponding toblocking 90 degrees or half of each half cycle of the rectified voltageV_(RECT). As shown in FIG. 4, transistor 475 may be a PNP transistorcoupled to operate in the linear region of the transistor. That is, thecompensation current I_(COMP) generated by compensation circuit 460 issubstantially proportional to the feedback signal U_(FB), between whenthe feedback signal U_(FB) has exceeded the phase angle voltagethreshold and when transistor 475 enters a linear region. In otherwords, the compensation circuit 460 increases the compensation currentI_(COMP) as the feedback signal U_(FB) decreases during a certain range.

Also shown in FIG. 4 is summing node 497, which may be one example ofsummer 165 in FIG. 1, is coupled to add the compensation currentI_(COMP) 406 to the peak input voltage signal I_(VINPK) and to providethe compensated peak input voltage signal I_(CVINPK) to controller 155of integrated control circuit 480.

In one embodiment, power supply 400 and LED array 305 are packagedtogether into a single apparatus, such as an LED lamp (i.e., LED lightbulb). The LED lamp including power supply 400 and LED array 305 may bedesigned to be interchangeable with and serve as a replacement forconventional incandescent or compact fluorescent light bulbs.

The operation of power supply 400 will now be described with referenceto FIG. 4 and FIG. 5. In a process block 505, controller 155 regulatesoutput current output current I_(OUT) in response to feedback signalU_(FB) and peak input voltage signal I_(VINPK). In process block 505 noinput from dimming control input 104 (see FIG. 1) has been received todim the output of LED array 305. Thus, compensation circuit 460 does notoutput a compensation current I_(COMP) 406 and the compensated inputvoltage signal I_(CVIN) 408 is equal to peak input voltage signalI_(VINPK). In a decision block 510, if a dimming control input 104 isreceived, process 500 advances to a process block 515 where outputsensor 450 senses the output voltage V_(OUT) and in process block 520where output sensor generates feedback signal U_(FB) in response to thesensed output voltage. In a decision block 525, if feedback signalU_(FB) is not below the phase angle voltage threshold the process 500returns to process block 505 and no compensation current is generated.In one example, feedback signal U_(FB) reaching the phase angle voltagethreshold corresponds with the phase angle of the blocked portion ofrectified voltage V_(RECT) exceeding the phase angle threshold.

If, in decision block 525, the feedback signal U_(FB) is below the phaseangle voltage threshold then process 500 advances to process block 530where compensation circuit 460 generates the compensation currentI_(COMP). In a process block 535, summing node 497 adds the compensationcurrent I_(COMP) to the peak input voltage signal I_(VINPK) to generatethe compensated peak input voltage signal I_(CVINPK). Next, in a processblock 540, controller 155 calculates the amount of power delivered tothe output of power converter 400 in response to the feedback currentI_(FB) and in response to the compensated peak input voltage signalI_(CVIN), to allow for natural dimming. In other words, adding thecompensation current I_(COMP) to peak input voltage signal I_(VINPK)allows the output current I_(OUT) to be adjusted in response to thedimming control input 104 when feedback signal drops below the phaseangle voltage threshold.

FIG. 6 is a functional block diagram of an LED array 305 and a switchingpower converter 600, in accordance with an embodiment of the presentinvention. Switching power converter 600 is illustrated as includingfilter capacitor 405, energy transfer element 120, input return 130,rectifier 135, output capacitor 140, output return 145, resistor 445,output sensor 450, a compensation circuit 660, peak input voltagedetector 470, integrated control circuit 480, bypass capacitor 485, anda node 697. Compensation circuit 660 is illustrated as including aresistor divider (i.e., resistors 655 and 665), a resistor 690, atransistor 675, and a node 695. Also shown in FIG. 6 are rectifiedvoltage V_(RECT), output voltage V_(OUT), output current I_(OUT), afeedback current I_(FB), a peak input voltage signal I_(VIN), acompensation current I_(COMP), and a compensated feedback currentI_(CFB).

Power converter 600 operates similarly to power converter 400 of FIG. 4.However, rather than adding compensation current I_(COMP) to peak inputvoltage signal I_(VINPK), compensation circuit 660 is coupled tosubtract compensation current I_(COMP) to compensate. In particular,compensation circuit 660 is coupled to sink a compensation currentI_(COMP) in response to the feedback signal U_(FB) being less than aphase angle voltage threshold (i.e., phase angle threshold reached). Inthe illustrated example, the value of resistors 655 and 665 are selectedto set the value of the phase angle voltage threshold, such that whenthe voltage at node 695 drops below the phase angle voltage threshold,transistor 675 turns on. When the phase angle voltage threshold isreached, the compensation current I_(COMP) flows through transistor 675.In one embodiment, the phase angle voltage threshold is representativeof a dimmer control input that corresponds to blocking 90 degrees ofeach half cycle of the rectified voltage V_(RECT). As shown in FIG. 6,transistor 675 may be a PNP transistor coupled to operate in the linearregion of the transistor. That is, the compensation current I_(COMP)generated by compensation circuit 660 is substantially proportional tofeedback signal U_(FB). Also, the compensation circuit 660 increases thecompensation current I_(COMP) as the feedback signal U_(FB) decreases.

Also shown in FIG. 6 is node 697 coupled to subtract the compensationcurrent I_(COMP) from the feedback current I_(p) and to provide thecompensated feedback current I_(CFB) to controller 155 of integratedcontrol circuit 480. In operation, when compensated feedback currentI_(CFB) is lowered, controller 155 delivers less power to output ofpower converter 600 thus counteracting the indication of power increaseof peak input voltage current I_(VINPK) when phase angle threshold isgreater than 90 degrees.

The operation of power converter 600 will now be described withreference to FIG. 6 and FIG. 7. In a process block 705, controller 155regulates output current I_(Our) in response to feedback signal U_(FB)and peak input voltage current I_(VINPK). In process block 705 when noinput from dimming control input 104 (see FIG. 1) has been received todim the output of LED array 305. Thus, compensation circuit 660 does notgenerate a compensation current I_(COMP) and the compensated feedbackcurrent I_(CFB) is the feedback current I_(FB). In a decision block 710,if a dimming control input 104 is received process 700 advances to aprocess block 715 where output sensor 450 senses the output voltageV_(OUT) and generates feedback signal U_(FB) in response to the sensedoutput voltage (i.e., process block 720). In a decision block 725, ifthe feedback signal U_(FB) is not below the phase angle voltagethreshold the process 700 returns to process block 705 and nocompensation current is generated.

If, in decision block 725, the feedback signal U_(FB) is below the phaseangle voltage threshold then process 700 advances to process block 730where compensation circuit 660 generates the compensation currentI_(COMP). In a process block 735, node 697 subtracts the compensationcurrent I_(COMP) from the feedback current I_(FB) to generate thecompensated feedback current I_(CFB). Next, in a process block 740,controller 155 calculates the amount of power delivered to the output ofpower converter 600 and maintains a constant output current I_(OUT) inresponse to the compensated feedback current I_(CFB) and in response topeak input voltage signal I_(VINPK), to allow for natural dimming. Thus,subtracting the compensation current I_(COMP) from the feedback currentI_(FB) allows the output current I_(OUT) to be adjusted in response tothe dimming control input 104 when feedback signal drops below a phaseangle voltage threshold.

While the invention herein disclosed has been described by means ofspecific embodiments, examples and applications thereof, numerousmodifications and variations could be made thereto by those skilled inthe art without departing from the scope of the invention set forth inthe claims.

What is claimed is:
 1. A power converter, comprising: an energy transferelement coupled to receive a rectified voltage having a non-blockedportion and a blocked portion, wherein an amount of the blocked portioncorresponds to a phase angle; a switch coupled to the energy transferelement; a controller coupled to control switching of the switch toregulate an output current of the power converter in response to aplurality of signals, wherein the plurality of signals includes a peakinput voltage signal and a feedback signal, the peak input voltagesignal representative of a peak input voltage of the power converter andthe feedback signal representative of the output voltage of the powerconverter; and a compensation circuit coupled to output a compensationsignal to adjust at least one of the plurality of signals in response tothe phase angle exceeding a phase angle threshold, and wherein thecompensation circuit does not output the compensation signal and doesnot adjust the at least one of the plurality of signals when the phaseangle is less than the phase angle threshold.
 2. The power converter ofclaim 1, wherein the phase angle threshold is 90 degrees.
 3. The powerconverter of claim 1, wherein the compensation circuit includes a phaseangle voltage threshold representative of the phase angle threshold,wherein the compensation circuit is coupled to adjust the at least oneof the plurality of signals in response to the feedback signal beingless than the phase angle voltage threshold.
 4. The power converter ofclaim 3, wherein the compensation signal is a compensation currentresponsive to the feedback signal when the feedback signal is less thanthe phase angle voltage threshold and wherein the power converterfurther comprises a summer coupled to add the compensation current tothe peak input voltage signal.
 5. The power converter of claim 4,wherein the summer is a summing node coupled to provide the controllerwith a compensated peak input voltage signal that is the compensationcurrent added to the peak input voltage signal.
 6. The power converterof claim 3, wherein the compensation signal is a compensation currentresponsive to the feedback signal when the feedback signal is less thanthe phase angle voltage threshold and wherein the compensation currentis coupled to subtract the compensation current from the feedbacksignal.
 7. The power converter of claim 3, wherein the compensationcircuit includes a resistor divider coupled to receive the feedbacksignal and to set the phase angle voltage threshold.
 8. The powerconverter of claim 7, wherein the compensation circuit includes atransistor coupled to the resistor divider and wherein the compensationcurrent flows through the transistor when the feedback signal is lessthan the phase angle voltage threshold.
 9. The power converter of claim1, further comprising an output sensor coupled to controller, whereinthe output sensor includes an auxiliary winding of the energy transferelement to generate the feedback signal.
 10. The power converter ofclaim 1, wherein the compensation signal is proportional to the feedbacksignal when the phase angle exceeds the phase angle threshold.
 11. Thepower converter of claim 1, wherein the switch and the controller areincluded in an integrated control circuit.
 12. An apparatus, comprising:a constant load; and a power converter coupled to provide an outputcurrent to the constant load, the power converter including: an energytransfer element coupled to receive a rectified voltage having anon-blocked portion and a blocked portion, wherein an amount of theblocked portion corresponds to a phase angle; a switch coupled to theenergy transfer element; a controller coupled to control switching ofthe switch to regulate the output current of the power converter inresponse to a plurality of signals, wherein the plurality of signalsincludes a peak input voltage signal and a feedback signal, the peakinput voltage signal representative of a peak input voltage of the powerconverter and the feedback signal representative of the output voltageof the power converter; and a compensation circuit coupled to output acompensation signal to adjust at least one of the plurality of signalsin response to the phase angle exceeding a phase angle threshold, andwherein the compensation circuit does not output the compensation signaland does not adjust the at least one of the plurality of signals whenthe phase angle is less than the phase angle threshold.
 13. Theapparatus of claim 12, wherein the phase angle threshold is 90 degrees.14. The apparatus of claim 12, wherein the compensation circuit includesa phase angle voltage threshold representative of the phase anglethreshold, wherein the compensation circuit is coupled to adjust the atleast one of the plurality of signals in response to the feedback signalbeing less than the phase angle voltage threshold.
 15. The apparatus ofclaim 14, wherein the compensation signal is a compensation currentresponsive to the feedback signal when the feedback signal is less thanthe phase angle voltage threshold and wherein the power converterfurther comprises a summer coupled to add the compensation current tothe peak input voltage signal.
 16. The apparatus of claim 15, whereinthe summer is a summing node coupled to provide the controller with acompensated peak input voltage signal that is the compensation currentadded to the peak input voltage signal.
 17. The apparatus of claim 14,wherein the compensation signal is a compensation current responsive tothe feedback signal when the feedback signal is less than the phaseangle voltage threshold and wherein the compensation current is coupledto subtract the compensation current from the feedback signal.
 18. Theapparatus of claim 14, wherein the compensation circuit includes: aresistor divider coupled to receive the feedback signal and to set thephase angle voltage threshold; and a transistor coupled to the resistordivider and wherein the compensation current flows through thetransistor when the feedback signal is less than the phase angle voltagethreshold.
 19. The apparatus of claim 12, further comprising an outputsensor coupled to controller, wherein the output sensor includes anauxiliary winding of the energy transfer element to generate thefeedback signal.
 20. The apparatus of claim 13, wherein the compensationsignal is proportional to the feedback signal when the phase angleexceeds the phase angle threshold.
 21. The apparatus of claim 13,wherein the switch and the controller are included in an integratedcontrol circuit.
 22. The apparatus of claim 13, wherein the constantload is an LED array.